318 PROFESSOR OWEN ON THE SKELETON 



impennis, I wrote for the loan of those of the left side, and was favoured by a prompt 

 and kind acquiescence, the bones being stated to be from a mature female bird. 



I have thus at command the materials for a description of the complete osteology of 

 this most rare and now generally regarded as extinct bird. 



In my ' Descriptive Catalogue of the Osteological Series contained in the Museum of 

 the Royal College of Surgeons of England,' ' I have briefly noticed characteristics of the 

 cranium, dorsal vertebrae, scapula, coracoid, femur, and tibia of an Alca impennis which 

 the founder of the collection, John Hunter, had succeeded in procuring ; Mr. Blyth 

 had previously made known the fact that the humerus " possessed a very small internal 

 cavity, while the tibia was completely filled with marrow"^; and these are the only 

 published notices of the osteology of the bird with which I am acquainted. 



§ 1. Vertebral column. 



In the present specimen there are twenty-two moveable vertebrae between the skull 

 and sacrum, the last nine supporting moveable ribs, of which the first two pairs have 

 free extremities ; the succeeding pairs of free pleurapophyses articulate with hsemapo- 

 physes, and these with the sternum. 



The sacrum appears to include fourteen vertebrae, of which the first supports a tenth 

 moveable pair of ribs, the last of the ordinary thoracic costal series : its haemapophysis 

 does not reach the sternum. 



The caudal vertebrae are fourteen in number, of which the last three are blended 

 together, and the first, by its pelvic relations, might claim to belong to the sacral 

 series. 



The centrum of the atlas, anchylosed as an odontoid process to the axis vertebra, 

 presents a pair of small facets for articulation with the posterior basal angles of its 

 proper neurapophyses ; but these are mainly supported by the hypapophysis, simulating 

 the body of the atlas, and with which they are confluent. The back part of the hypa- 

 pophysis oflfers a flat surface to the centrum of the axis, beneath which it is slightly 

 produced at its lower part, being here wedged into the notch between the true bodies of 

 the axis and atlas. The fore part of the hypapophysis combines with the neurapophyses 

 to form the cup for the condyle of the occiput ; the cup is emarginate above, and 

 traversed by the ligamentous continuation of the " odontoid " in its way to adhere to the 

 upper part of the occipital ball. The atlantal neurapophyses diverge as they rise, and 

 are joined together above by a broad plate slightly arching across from one to the other 

 neurapophysis. A process extends backward from each place of junction. There is 

 no neural spine. The neural canal has a wide transversely elliptical area. 



The body of the axis is elongate, compressed and carinate below, the keel being 

 slightly produced and curved at the hind part. The expanded anterior end of the 

 centrum has a small notch near or at the junction of the neurapophyses. These deve- 



' 4to, 1853, vol. i. p. 221, preps, nos. 1150-1160. ' Proc. Zool. Soc. November 14, 1837, p. 122. 



